Fix typo (#170)

Change "user strict" to the correct "use strict"

src/constants.ts

@@ -1,7 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
-"user strict";
+"use strict";
 import * as os from "os";
 import * as path from "path";
 import * as vscode from "vscode";